The exposure of proteins to ozone leads to rapid destruction of the aromatic and sulfur containing amino acid residues. The other amino acid residues are relatively resistant to oxidation by ozone. The sensitivity of a given aromatic amino acid residue to ozone is dependent upon the secondary, tertiary and quaternary structure. The oxidation of aromatic amino acid residues by ozone leads to a number of products including as yet unidentified carbonyl derivatives, and in the case of histidine to asparagine (or aspartic acid). The distribution of products is dependent upon the location of the aromatic amino acid in the polypeptide chain. Thus, asparagine accounted for 30, 60 and 100% of the histidine oxidation products when the histidine was present in the N-terminal, central or C-terminal position, respectively, in tripeptides containing 2 equivalents of alanine and only one of histidine.